The overall rate of a reaction is governed by

For composite reactions, the overall rate is determined by the slowest intermediate step.

$\mathrm{Ex}:\mathrm{Consider} \mathrm{the} \mathrm{reation} ,{\mathrm{NO}}_2+\mathrm{CO}\to \mathrm{NO}+{\mathrm{CO}}_2$

${\mathrm{NO}}_2+{\mathrm{NO}}_2\underset{\mathrm{Slow}}{\overset{{\mathrm{K}}_1}{\to }}{\mathrm{NO}}_3+\mathrm{NO}---\left(1\right)$

${\mathrm{NO}}_3+\mathrm{CO}\underset{\mathrm{Slow}}{\overset{{\mathrm{K}}_1}{\to }}{\mathrm{NO}}_2{\mathrm{CO}}_2$

${\mathrm{NO}}_2+\mathrm{CO}\to \mathrm{NO}+{\mathrm{CO}}_2$

Thus, rate will be determined by slow step 1 and

$\mathrm{Rate}=\mathrm{K}{\left[{\mathrm{NO}}_2\right]}^2$